Reproductions of multi-colored subject matter are commonly printed by processes which may vary as to the specific printing techniques or equipment employed, but which have in common the fact that the reproduced work pieces are printed in stages with separate impressions for the components of the subject matter corresponding to each of a selected plurality of colors being imprinted upon the stock successively. The problem of maintaining proper registration between the successive impressions involved in such printing is well known, and much has been done to solve that problem by modern printing technology. However, the companion problem inherent in successive-impression, multi-color printing, that of preserving the fidelity of color reproduction to the quality standards required in producing fine art prints, greeting cards, color-sensitive advertising illustrations and the like, has long resisted the provision of any truly practical and economical solution. It is this latter problem to which the present invention is directed.
Because of the relatively high cost of the stock normally employed in multi-color printing, as well as the time and effort required to prepare a press for properly registered printing of each color impression, it has long been customary to use large dimension stock and to simultaneously print thereon an appropriate lay-out of different graphic subjects, which can then be separated by cutting after the printing is completed. The diversity of the areas of appearance of particular color components in the various subjects to be printed on each piece of stock, the size of the presses needed for handling the large stock, and the manner in which color hues are to be provided in the reproduction from the effects of overprinting impressions of different selected colors, all contribute to the criticality and difficulty of establishing and maintaining uniform color characteristics of desired density throughout the expanse of each successive color impression.
In practical terms, the matter of such color quality control ultimately translates into careful adjustment by the pressman of the multiplicity of ink feed, water feed and other controls affecting the density of the color printed at various zones across the stock by each press or color run until a work piece exhibiting the desired color density and uniformity across the stock is produced, and this is required at every stage of the production process from initial set-up for individual color proofing, through multi-color proofing to obtain an "OKed proof", to final printing in which the approved color characteristics of the OKed proof must be substantially duplicated for every color involved on a continuing basis throughout the production run.
The real gist of the problem, however, concerns what guidance can be provided for the pressman in making such adjustments both accurately and expeditiously in an environment where mere undue delay may cause a change in the characteristics of the printed product sufficient to render it unsatisfactory. Both because multi-color printing usually involves a substantial degree of successive overprinting of the selected colors and for the sake of convenience otherwise, it has for some time been customary practice for the impressions imprinted for each color to include a number of spots or blocks of such color printed adjacent the primary subject matter being reproduced and so arranged as to be offset from such color blocks for the other colors. Current practice arranges such blocks in alignment across the stock to present what is often called a color control strip, which includes sufficient blocks of each color spaced across the stock to provide the pressman with a reliable indication of the color densities for each color at a sufficient number of zones across the stock to determine uniformity or the need for adjustment of the appropriate color-affecting controls on the press applicable to individual zones.
Although earliest practice involved attempted utilization of such color control blocks through visual evaluation, either by mere judgment of the skilled pressman having a "good eye for color" or by visual comparison of such blocks with color samples, such method is simply not sufficiently accurate or reliable for high quality, multi-color work.
With a view to overcoming the inaccuracies of visual color judgements or comparisons, the reflection color densitometer was developed and has enjoyed wide usage. Basically, such instruments have involved an optical assembly for picking up and selectively filtering light reflected from a very small area such as a color control block and for converting the intensity of the light so picked up into an electrical signal whose strength could be indicated on a meter or the like as a measure of color density. Such devices have typically included a hand-held pick-up head requiring careful manual positioning over each color control block to be sensed optically, and the reflected light has then been coupled through a flexible fiber-optics "cable" to a housing containing some provision for changing filters under operator control. The light intensity through the filter has then been sensed, amplified, and converted to a density measurement by appropriate circuitry and a meter or other indicating component.
In recent years, digital type indicating components have replaced ordinary electrical meters, and some densitometer instruments have also been equipped for printing out a permanent written record of density measurements under operator control. In general, however, such popularly employed, manual head, color densitometers, although the best thing long available for the purpose, have suffered from various disadvantages inherent in their constructions and the techniques required for their use, including the time and effort required to manually position the pick-up head over each color control spot, to manually control the selection of a filter appropriate for the particular color of the control block to be sensed, and to attempt to accurately correlate the measurement data produced with the appropriate zones and colors of the work piece as a basis for determining necessary press adjustments.
Most recently, a type of color densitometer has been produced in Europe, which attempted to alleviate some of the mentioned problems by mounting the densitometer head on a movable carriage for mechanically stepped advancement along the color control strip to successively juxtapose with individual blocks of the latter and by providing for the mechanical changing of filters in response to electrical trigger signals. Although such last-mentioned equipment did tend to relieve the operator of the need for manually juxtaposing the pick-up head with each control block and did facilitate the changing of filters in response to a pre-established control, it has not gained wide spread acceptance as any real solution to the overall problem, since the method employed for controlling various functions of the equipment created new problems and inflexibility. More specifically, such last known prior effort to solve the problem contemplated and required, as the initial step for establishing a means for accomplishing what most accurately might be termed "semi-automatic" control over the densitometer system functions, the preparation of a punched tape control medium carrying a sequence of codes which could be successively sensed by a punched tape reader and decoded to provide a sequence of electrical control signals for effecting stepped advancement of the carriage bearing the pick-up head, changing of the filter, and triggering of a device to print or punch measurement data on another paper tape. The initial preparation of such control tapes with the use of a manual or keyboard actuated perforator, however, proved to be a burdensome task. Moreover, each such control tape was essentially limited in use to only the completely imprinted work piece for a particular printing job or ones substantially identical thereto, so that a different control tape had to be prepared for every substantially differing job or stage thereof to be checked. But, perhaps most significantly, the control of the color density checking process and the system functions required therefor in the mentioned method was still left to depend upon the accuracy of the manually generated control tape and to remain inflexibly fixed to the particular control sequence selected and crystallized beforehand by a human operator as what should be appropriate for the particular job.